The XM-23 sealant for space application was studied carefully. The application of it in the mirror subassembly of a space optical remote sensor as a damping agent was mainly considered. Its composition and function were analyzed. The mass ratio of each component was selected reasonably and the adding sequence of each composition in the sealant mixing process was optimized. Finally, an ideal curing process route for sealing the mirror subassembly of a space optical remote sensor in micro-stress was obtained. The testing result showed that the process route had the features of less time, less internal stress and reasonable composition ratio. It met the micro-stress requirements of the sealant used for space application. Therefore, it could be widely used in the structures for space application.